The exhaust passage of a diesel engine is sometimes fitted with a lean NOx catalyst (referred to as LNC hereinafter) for reducing and decreasing nitrogen oxides (referred to as NOx hereinafter), which are particularly generated in a large amount in lean combustion, from the exhaust gas.
The LNC functions to trap (more specifically adsorb) NOx in lean combustion where the oxygen concentration in the exhaust gas is relatively high, and the trapped NOx is reduced into a harmless form and discharged to the atmosphere in rich combustion where the concentration of unburnt components in the exhaust gas is relatively high. The NOx purification ability of the LNC tends to decrease as the amount of trapped NOx increases, and therefore, a control is conducted to make the combustion condition rich from time to time to release and reduce the NOx trapped by the LNC.
Meanwhile, because the fuel includes sulfur contents, sulfur oxides (SOx) and hydrogen sulfides (H2S) are also emitted from the combustion chamber. Such sulfur contents are also adsorbed by the LNC (this state is referred to as sulfur poisoning hereinafter) in the same way as for NOx, and the capability of LNC to adsorb NOx diminishes as the sulfur poisoning proceeds. Therefore, it is necessary to release or remove the sulfur contents adsorbed by the LNC from time to time. In order to carry out the process of releasing sulfur contents from the LNC (referred to as sulfur purging hereinafter), it is necessary to achieve both a prescribed temperature and a prescribed exhaust air fuel ratio (referred to as exhaust A/F hereinafter) in the LNC. As a technique for this, it is known to perform a post-combustion supplementary fuel injection (referred to as post-injection hereinafter) in addition to the main fuel injection conducted during the intake stroke, to thereby make the exhaust A/F rich and raise the LNC temperature so as to be higher than a prescribed value (see Japanese Patent Application Publication (kokai) No. 9-32619, for example).
In such a conventional technique described in JPA Publication No. 9-32619, a feedback control is conducted mainly based on the LNC temperature. However, in a case where an additional exhaust gas processing device other than the LNC is provided in the exhaust system, an activation temperature as well as a detrimental temperature range can be different between the LNC and the additional exhaust gas processing device, and therefore, the exhaust A/F control solely based on the LNC temperature may not be able to maintain an environment favorable to the additional exhaust gas processing device.